The DC/AC inverter has been the major essentials for implementing the uninterrupted power supply (UPS), AC power sources, and any kind of the AC motor drivers etc. Due to the rapid progress in the development of the power electronics, many approaches for realizing a compact inverter with high efficiency have been proposed.
In year 1988, the phase-controlled DC/AC Inverter has been proposed for using the differences of phase angles between different switches to modulate the output voltage. The above-mentioned phase-controlled DC/AC Inverter has relatively a simpler configuration, a higher power efficiency, a minimized volume of the isolation transformer due to the high-frequency switching so as to be beneficial to the manufacturing costs and the space limitations. However, the total harmonic distortions (THD) of the output voltages of this kind of inverters are relatively larger.
Nowadays, the widely used control strategies for realizing a DC/AC inverter is the pulse width modulation (PWM) for using an input reference signal to compare with a triangular waveform so as to produce a width-modulated pulse to fulfill the object of controlling output voltage waveforms. In general, a voltage source inverter will have an output voltage of a rectangular waveform with a higher harmonic so as to decrease the power efficiency, therefore, the PWM inverter circuits can be employed to solve the problems regarding the voltage modulations and the waveforms distortions. The PWM is mainly focused on the control of the on/off of the power switches of the inverter circuits so as to get a series of pulses with equivalent-amplitudes but with nonequivalent-widths thus the widths of the pulses will form a sinusoidal distribution according to the Fourier series and the combinations of the pulses will form an equivalent sinusoidal voltage waveforms so as to engage in the modulation of each pulse in accordance with certain rules to change the magnitudes of the output voltages as well as the frequencies.
Due to the triangular waves are employed to compare with the reference signals in the above-mentioned PWM, it is not easy to decrease the harmonic distortions of AC outputs, thus other circuits are employed so as to gain the better characteristics of the PWM controlling method yet the complexity of the circuit configuration is increased accordingly.
Furthermore, a DC/AC inverter with outputs of any kind of waveforms might be the demands in the future so as to cope with the progress of the advanced technologies. Actually, a DC/AC inverter with the programmable output waveforms and frequencies could not be realized by the previously proposed PWM strategies.
Keep the drawbacks of the prior arts in mind, and employ experiments and research full-heartily and persistently, the sigma-delta modulation inverter with programmable waveform output is finally conceived by the applicants.